There has been proposed a traction control system configured to reduce a vehicle engine driving power in order to restore drive wheel traction for the vehicle with respect to a road surface when a slip occurs in the drive wheel (see e.g., Japanese Laid-Open Patent Application Publication No. Hei. 7-103009). According to this control system, when an increase rate of an engine speed becomes larger than a predetermined threshold, an ignition retard control is used so that an ignition timing of the engine is retarded with respect to an optimal ignition timing to thereby reduce engine driving power. Thus, further slip is inhibited.
This traction control system is able to detect a rapid increase in the engine speed which is caused by abrupt spinning of the drive wheel, for example, when the vehicle is traveling over a crack in a road surface, but has difficulty in distinguishing between the slip and an engine noise in a normal state, where the increase rate in the engine speed does not rapidly rise during continued and gradual sliding of the drive wheel, for example, when the vehicle is traveling around a corner at a low speed. Therefore, it is advantageous to set a threshold to turn on and off traction control based on a change in the increase rate of the engine speed. Whereas the threshold may be easily set when the change in the increase rate is large, it is difficult to set a threshold for distinguishing between the slip and the engine noise when the change in the increase rate is small, which may result in a false slip detection. This decreases precision in the traction control.